Sweet pickles for home canning

The purpose of using a low simmer during stock-making is to avoid agitating the liquid. You want the fat to settle on top, but a rapid boil will simply disperse the fat and other impurities all throughout the stock, resulting in - as you've hinted at - a cloudy texture and possibly an inferior taste.

The other reason to avoid boiling is simply to prevent the stock from reducing too much. If you're simmering for 6 hours, you could lose all the water. If you leave the stock unattended during that time, and the water level drops too low, then you're not getting anything out of it; the water needs to be covering the bones in order to break down the collagen.

All that said, once you are finished with stock, you should have skimmed it and strained it through a fine sieve and probably a few layers of cheesecloth as well. If you've made your stock correctly, it will be clear and have (almost) no fat. Boiling it briefly at that point isn't really going to harm it; there's not enough fat left to cloud the stock and you're not boiling it for long enough to really reduce it.

So I would say go ahead and don't worry about the temperature. As far as I know, nothing magical happens at 180° F, that's just a rough temperature guideline for "low simmer".

In answer to your second part, no. If something is not acidic enough for water canning it seems it doesn't matter how long you process it, it isn't safe. This is the reasoning all the cookbooks I've read provide for adding a teaspoon of lemon juice to each pint of tomatoes.

I would therefore use this strategy:

• get things to the right pH for water or pressure canning as a separate first step. (Fruit will generally be ok, tomatoes need lemon juice, pickles will be ok, etc)
• check the processing time for pints and quarts of various things (jams, fruit, pickles etc) and look for the pattern: is it consistently 25% more time, or 5 extra minutes? Work out the pattern.
• repeat for hot vs cold pack: do you halve the time, or subtract 10 minutes, or what?

Now armed with a hot-pack pints recipe, you can adjust it for cold-pack quarts or vice versa. You should also, in poring through these recipes and charts, have come to understand various categories (fruit, jam, pickles, nonpickled vegetables etc) and be familiar with what times they need. Then faced with a chutney, conserve, or pickle that isn't on anybody's list, you should be able to choose what category it most likely belongs to, and safely pick a time for it.